Techniques for perforating and fracturing a formation surrounding a borehole are known in the art. The most common technique for perforating and fracturing a formation to stimulate production includes the steps of: 1) penetrating a production zone with a projectile; and 2) hydraulically pressurizing the borehole to expand or propagate a perforated hole into a fracture. This technique proves to be extremely expensive due to the preparation required for pressurizing a portion of a borehole. Typically, pressure around a production zone in the borehole is increased by pumping fluids into that portion of the well to obtain the high pressures necessary to expand the fracture in the production zones. This operation is generally time intensive and costly making these techniques unattractive for either multiple zone wells or wells with a low rate of production.
Perforations are done generally underbalanced, a lower wellbore pressure than reservoir pressure being applied to flush the tunnel just after the perforating. However, the perforation efficiency can be unpredictable with an underbalanced technique, the perforation efficiency being altered by perforation damage (crushed zone around the runnel) and/or the required underbalance to clean the perforation tunnel is unachievable. A review revealed inconsistent results achieved with underbalanced perforating, with an average perforation efficiency of less than 25%. In overbalanced perforating, a high pressure is applied to the cased wellbore before the shooting of the perforations. Higher production rates and negative productivity skin have been reported. For instance, 14 wells recently perforated with this technique showed a median negative completion skin factor of -2.0. Another reason to use an overbalance treatment comes from the problems of fracture initiation and fracture link-up which are dependent on the near wellbore stresses. It has been shown that the fracture opening surface is not generally a smooth surface and that there are a large amount of rock masses entrapped by connecting cracks. This situation could be greatly improved with overbalanced perforating which shall favor fracture link-up from each individual perforations. Overbalance-generated fracture may also initially align with the perforation axis.
Less expensive techniques using gas propellants have been implemented in place of hydraulic fracture propagation. The resulting procedure is similar to that discussed above. First, a projectile is fired to penetrate the production zone. Second, a propellant device is ignited to pressurize the zone of interest to initiate and propagate the fracture.
Godfrey et al., U.S. Pat. No. 4,039,030, describes a method using a propellant to maintain the pressure caused by a high explosive charge over a longer period. The high explosives are used to generate fractures while the role of the propellant is to extend these fractures. In accordance with this technique, the casing must be perforated prior to ignition of the high explosives and propellant as the high explosives are used exclusively to fracture the formation but not to perforate the casing.
Ford et al., U.S. Pat. No. 4,391,337, describes tin integrated perforation and fracturing device in which a high velocity penetrating jet is instantaneously followed by a high pressure gas propellant. In essence, a tool including propellant gas generating materials and shaped charges is positioned in a desired zone in the borehole. The penetrating shaped charges and propellant material are ignited simultaneously. The high pressure propellant material amplifies and propagates the fractures initiated by the shaped charges.
Dees et al., U.S. Pat. No. 5,131,472, and Schmidt et al., U.S. Pat. No. 5,271,465 concern overbalance perforating and stimulation methods which employ a long gas section of tubing or casing to apply high downhole pressure. Fluid is pumped downhole until the pressure in the tubing reaches a pressure above the fracture pressure of the formation. A perforating gun is then fired to perforate the casing. Because the applied pressure is enough to break the formation, fractures propagate into the formation. The gas column forces the fluid into the fractures and propagates them. Two issues can limit the use of this technique. First, the wellhead pressure must be compatible with safety limits. Generally the wellhead pressure is limited to 10,000 psi, and the required bottomhole pressure may not be achievable. As a rule of thumb overpressured well fracturing job should be done at 1.4 psi/ft. Specifically, the minimum applied pressure gradient is the formation fracturing gradient plus 0.4 psi/ft. Therefore, this overbalance technique can be unusable in deep wells. To increase the bottomhole pressure, a long section of the tubing is generally filled with liquid: a length of 1,000 m of liquid section is typical. However, this solution leads to the second limitation of the technique, mainly that long liquid section in small OD tubing will generate important friction losses due to the movement of the liquid inside the tubing. Friction losses will lower fracture propagation speed. As a result, leakage of the fracture fluid into the formation (seepage losses) through the fracture walls are much more important and the fracture extension can be greatly reduced.
In Hill, U.S. Pat. No. 4,823,875, the well casing is filled with a compressible hydraulic fracturing fluid comprising a mixture of liquid, compressed gas, and proppant material. The pressure is raised to about 1000 psi greater than the pressure of the zone to be fractured by pumping fluid downhole. The gas generating units are simultaneously ignited to generate combustion gasses and perforate the well casing. The perforated zone is fractured by the rapid outflow of an initial charge of sand-free combustion gas at the compression pressure followed by a charge of fracturing fluid laden with proppant material and then a second charge of combustion gas.
Although the prior art suggests downhole gas generators for use in fracturing operations, none drive a liquid column. These prior techniques have not proven attractive from an economical or technical point of view. In conventional hydraulic fracturing, even with the use of downhole propellant gas generators, a substantial amount of hydraulic power capability must maintained at the surface. None of the techniques have provided an economical process for perforating and fracturing as part of a single highly efficient operation.